Reconfigurable optical add-drop multiplexers with servo control and dynamic spectral power management capabilities

ABSTRACT

This invention provides a novel wavelength-separating-routing (WSR) apparatus that uses a diffraction grating to separate a multi-wavelength optical signal by wavelength into multiple spectral channels, which are then focused onto an array of corresponding channel micromirrors. The channel micromirrors are individually controllable and continuously pivotable to reflect the spectral channels into selected output ports. As such, the inventive WSR apparatus is capable of routing the spectral channels on a channel-by-channel basis and coupling any spectral channel into any one of the output ports. The WSR apparatus of the present invention may be further equipped with servo-control and spectral power-management capabilities, thereby maintaining the coupling efficiencies of the spectral channels into the output ports at desired values. The WSR apparatus of the present invention can be used to construct a novel class of dynamically reconfigurable optical add-drop multiplexers (OADMs) for WDM optical networking applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/005,714, filed Nov. 7, 2001 now U.S. Pat. No. 6,687,431, which is acontinuation of U.S. application Ser. No. 09/938,426, filed Aug. 23,2001, now U.S. Pat No. 6,625,346 which claims the benefit of U.S.application Ser. No. 60/277,217, filed Mar. 19, 2001.

FIELD OF THE INVENTION

This invention relates generally to optical communication systems. Morespecifically, it relates to a novel class of dynamically reconfigurableoptical add-drop multiplexers (OADMs) for wavelength divisionmultiplexed optical networking applications.

BACKGROUND

As fiber-optic communication networks rapidly spread into every walk ofmodern life, there is a growing demand for optical components andsubsystems that enable the fiber-optic communications networks to beincreasingly scalable, versatile, robust, and cost-effective.

Contemporary fiber-optic communications networks commonly employwavelength division multiplexing (WDM), for it allows multipleinformation (or data) channels to be simultaneously transmitted on asingle optical fiber by using different wavelengths and therebysignificantly enhances the information bandwidth of the fiber. Theprevalence of WDM technology has made optical add-drop multiplexersindispensable building blocks of modern fiber-optic communicationnetworks. An optical add-drop multiplexer (OADM) serves to selectivelyremove (or drop) one or more wavelengths from a multiplicity ofwavelengths on an optical fiber, hence taking away one or more datachannels from the traffic stream on the fiber. It further adds one ormore wavelengths back onto the fiber, thereby inserting new datachannels in the same stream of traffic. As such, an OADM makes itpossible to launch and retrieve multiple data channels (eachcharacterized by a distinct wavelength) onto and from an optical fiberrespectively, without disrupting the overall traffic flow along thefiber. Indeed, careful placement of the OADMs can dramatically improvean optical communication network's flexibility and robustness, whileproviding significant cost advantages.

Conventional OADMs in the art typically employmultiplexers/demultiplexers (e.g, waveguide grating routers orarrayed-waveguide gratings), tunable filters, optical switches, andoptical circulators in a parallel or serial architecture to accomplishthe add and drop functions. In the parallel architecture, as exemplifiedin U.S. Pat. No. 5,974,207, a demultiplexer (e.g., a waveguide gratingrouter) first separates a multi-wavelength signal into its constituentspectral components. A wavelength switching/routing means (e.g., acombination of optical switches and optical circulators) then serves todrop selective wavelengths and add others. Finally, a multiplexercombines the remaining (i.e., the pass-through) wavelengths into anoutput multi-wavelength optical signal. In the serial architecture, asexemplified in U.S. Pat. No. 6,205,269, tunable filters (e.g., Braggfiber gratings) in combination with optical circulators are used toseparate the drop wavelengths from the pass-through wavelengths andsubsequently launch the add channels into the pass-through path. And ifmultiple wavelengths are to be added and dropped, additionalmultiplexers and demultiplexers are required to demultiplex the dropwavelengths and multiplex the add wavelengths, respectively.Irrespective of the underlying architecture, the OADMs currently in theart are characteristically high in cost, and prone to significantoptical loss accumulation. Moreover, the designs of these OADMs are suchthat it is inherently difficult to reconfigure them in a dynamicfashion.

U.S. Pat. No. 6,204,946 to Askyuk et al. discloses an OADM that makesuse of free-space optics in a parallel construction. In this case, amulti-wavelength optical signal emerging from an input port is incidentonto a ruled diffraction grating. The constituent spectral channels thusseparated are then focused by a focusing lens onto a linear array ofbinary micromachined mirrors. Each micromirror is configured to operatebetween two discrete states, such that it either retroreflects itscorresponding spectral channel back into the input port as apass-through channel, or directs its spectral channel to an output portas a drop channel. As such, the pass-through signal (i.e., the combinedpass-through channels) shares the same input port as the input signal.An optical circulator is therefore coupled to the input port, to providenecessary routing of these two signals. Likewise, the drop channelsshare the output port with the add channels. An additional opticalcirculator is thereby coupled to the output port, from which the dropchannels exit and the add channels are introduced into the output port.The add channels are subsequently combined with the pass-through signalby way of the diffraction grating and the binary micromirrors.

Although the aforementioned OADM disclosed by Askyuk et al. has theadvantage of performing wavelength separating and routing in free spaceand thereby incurring less optical loss, it suffers a number oflimitations. First, it requires that the pass-through signal share thesame port/fiber as the input signal. An optical circulator therefore hasto be implemented, to provide necessary routing of these two signals.Likewise, all the add and drop channels enter and leave the OADM throughthe same output port, hence the need for another optical circulator.Moreover, additional means must be provided to multiplex the addchannels before entering the system and to demultiplex the drop channelsafter exiting the system. This additional multiplexing/demultiplexingrequirement adds more cost and complexity that can restrict theversatility of the OADM thus-constructed. Second, the opticalcirculators implemented in this OADM for various routing purposesintroduce additional optical losses, which can accumulate to asubstantial amount. Third, the constituent optical components must be ina precise alignment, in order for the system to achieve its intendedpurpose. There are, however, no provisions provided for maintaining therequisite alignment; and no mechanisms implemented for overcomingdegradation in the alignment owing to environmental effects such asthermal and mechanical disturbances over the course of operation.

U.S. Pat. No. 5,906,133 to Tomlinson discloses an OADM that makes use ofa design similar to that of Aksyuk et al. There are input, output, dropand add ports implemented in this case. By positioning the four ports ina specific arrangement, each micromirror, notwithstanding switchablebetween two discrete positions, either reflects its correspondingchannel (coming from the input port) to the output port, orconcomitantly reflects its channel to the drop port and an incident addchannel to the output port. As such, this OADM is able to perform boththe add and drop functions without involving additional opticalcomponents (such as optical circulators used in the system of Aksyuk etal.). However, because a single drop port is designated for all the dropchannels and a single add port is designated for all the add channels,the add channels would have to be multiplexed before entering the addport and the drop channels likewise need to be demutiplexed upon exitingfrom the drop port. Moreover, as in the case of Askyuk et al., there areno provisions provided for maintaining requisite optical alignment inthe system, and no mechanisms implemented for combating degradation inthe alignment due to environmental effects over the course of operation.

As such, the prevailing drawbacks suffered by the OADMs currently in theart are summarized as follows:

-   1) The wavelength routing is intrinsically static, rendering it    difficult to dynamically reconfigure these OADMs.-   2) Add and/or drop channels often need to be multiplexed and/or    demultiplexed, thereby imposing additional complexity and cost.-   3) Stringent fabrication tolerance and painstaking optical alignment    are required. Moreover, the optical alignment is not actively    maintained, rendering it susceptible to environmental effects such    as thermal and mechanical disturbances over the course of operation.-   4) In an optical communication network, OADMs are typically in a    ring or cascaded configuration. In order to mitigate the    interference amongst OADMs, which often adversely affects the    overall performance of the network, it is essential that the power    levels of spectral channels entering and exiting each OADM be    managed in a systematic way, for instance, by introducing power (or    gain) equalization at each stage. Such a power equalization    capability is also needed for compensating for non-uniform gain    caused by optical amplifiers (e.g., erbium doped fiber amplifiers)    in the network. There lacks, however, a systematic and dynamic    management of the power levels of various spectral channels in these    OADMs.-   5) The inherent high cost and heavy optical loss further impede the    wide application of these OADMs.

In view of the foregoing, there is an urgent need in the art for opticaladd-drop multiplexers that overcome the aforementioned shortcomings in asimple, effective, and economical construction.

SUMMARY

The present invention provides a wavelength-separating-routing (WSR)apparatus and method which employ an array of fiber collimators servingas an input port and a plurality of output ports; awavelength-separator; a beam-focuser; and an array of channelmicromirrors.

In operation, a multi-wavelength optical signal emerges from the inputport. The wavelength-separator separates the multi-wavelength opticalsignal into multiple spectral channels, each characterized by a distinctcenter wavelength and associated bandwidth. The beam-focuser focuses thespectral channels into corresponding spectral spots. The channelmicromirrors are positioned such that each channel micromirror receivesone of the spectral channels. The channel micromirrors are individuallycontrollable and movable, e.g., continuously pivotable (or rotatable),so as to reflect the spectral channels into selected ones of the outputports. As such, each channel micromirror is assigned to a specificspectral channel, hence the name “channel micromirror”. And each outputport may receive any number of the reflected spectral channels.

A distinct feature of the channel micromirrors in the present invention,in contrast to those used in the prior art, is that the motion, e.g.,pivoting (or rotation), of each channel micromirror is under analogcontrol such that its pivoting angle can be continuously adjusted. Thisenables each channel micromirror to scan its corresponding spectralchannel across all possible output ports and thereby direct the spectralchannel to any desired output port.

In the WSR apparatus of the present invention, the wavelength-separatormay be provided by a ruled diffraction grating, a holographicdiffraction grating, an echelle grating, a curved diffraction grating, adispersing prism, or other wavelength-separating means known in the art.The beam-focuser may be a single lens, an assembly of lenses, or otherbeam-focusing means known in the art. The channel micromirrors may beprovided by silicon micromachined mirrors, reflective ribbons (ormembranes), or other types of beam-deflecting means known in the art.And each channel micromirror may be pivotable about one or two axes. Thefiber collimators serving as the input and output ports may be arrangedin a one-dimensional or two-dimensional array. In the latter case, thechannel micromirrors must be pivotable biaxially.

The WSR apparatus of the present invention may further comprise an arrayof collimator-alignment mirrors, in optical communication with thewavelength-separator and the fiber collimators, for adjusting thealignment of the input multi-wavelength signal and directing thespectral channels into the selected output ports by way of angularcontrol of the collimated beams. Each collimator-alignment mirror may berotatable about one or two axes. The collimator-alignment mirrors may bearranged in a one-dimensional or two-dimensional array. First and secondarrays of imaging lenses may additionally be optically interposedbetween the collimator-alignment mirrors and the fiber collimators in atelecentric arrangement, thereby “imaging” the collimator-alignmentmirrors onto the corresponding fiber collimators to ensure an optimalalignment.

The WSR apparatus of the present invention may further include aservo-control assembly, in communication with the channel micromirrorsand the output ports. The servo-control assembly serves to monitor thepower levels of the spectral channels coupled into the output ports andfurther provide control of the channel micromirrors on an individualbasis, so as to maintain a predetermined coupling efficiency of eachspectral channel in one of the output ports. As such, the servo-controlassembly provides dynamic control of the coupling of the spectralchannels into the respective output ports and actively manages the powerlevels of the spectral channels coupled into the output ports. (If theWSR apparatus includes an array of collimator-alignment mirrors asdescribed above, the servo-control assembly may additionally providedynamic control of the collimator-alignment mirrors.) Moreover, theutilization of such a servo-control assembly effectively relaxes therequisite fabrication tolerances and the precision of optical alignmentduring assembly of a WSR apparatus of the present invention, and furtherenables the system to correct for shift in optical alignment over thecourse of operation. A WSR apparatus incorporating a servo-controlassembly thus described is termed a WSR-S apparatus, thereinafter in thepresent invention.

Accordingly, the WSR-S (or WSR) apparatus of the present invention maybe used to construct a variety of optical devices, including a novelclass of dynamically reconfigurable optical add-drop multiplexers(OADMs), as exemplified in the following embodiments.

One embodiment of an OADM of the present invention comprises anaforementioned WSR-S (or WSR) apparatus and an optical combiner. Theoutput ports of the WSR-S apparatus include a pass-through port and oneor more drop ports, each carrying any number of the spectral channels.The optical combiner is coupled to the pass-through port, serving tocombine the pass-through channels with one or more add spectralchannels. The combined optical signal constitutes an output signal ofthe system. The optical combiner may be an N×1 (N≧2) broadbandfiber-optic coupler, for instance, which also serves the purpose ofmultiplexing a multiplicity of add spectral channels to be coupled intothe system.

In another embodiment of an OADM of the present invention, a first WSR-S(or WSR) apparatus is cascaded with a second WSR-S (or WSR) apparatus.The output ports of the first WSR-S (or WSR) apparatus include apass-through port and one or more drop ports. The second WSR-S (or WSR)apparatus includes a plurality of input ports and an exiting port. Theconfiguration is such that the pass-through channels from the firstWSR-S apparatus and one or more add channels are directed into the inputports of the second WSR-S apparatus, and consequently multiplexed intoan output multi-wavelength optical signal directed into the exiting portof the second WSR-S apparatus. That is to say that in this embodiment,one WSR-S apparatus (e.g., the first one) effectively performs a dynamicdrop function, whereas the other WSR-S apparatus (e.g., the second one)carries out a dynamic add function. And there are essentially nofundamental restrictions on the wavelengths that can be added ordropped, other than those imposed by the overall communication system.Moreover, the underlying OADM architecture thus presented isintrinsically scalable and can be readily extended to any number of theWSR-S (or WSR) systems, if so desired for performing intricate add anddrop functions in a network environment.

Those skilled in the art will recognize that the aforementionedembodiments provide only two of many embodiments of a dynamicallyreconfigurable OADM according to the present invention. Various changes,substitutions, and alternations can be made herein, without departingfrom the principles and the scope of the invention. Accordingly, askilled artisan can design an OADM in accordance with the presentinvention, to best suit a given application.

All in all, the OADMs of the present invention provide many advantagesover the prior art devices, notably:

-   1) By advantageously employing an array of channel micromirrors that    are individually and continuously controllable, an OADM of the    present invention is capable of routing the spectral channels on a    channel-by-channel basis and directing any spectral channel into any    one of the output ports. As such, its underlying operation is    dynamically reconfigurable, and its underlying architecture is    intrinsically scalable to a large number of channel counts.-   2) The add and drop spectral channels need not be multiplexed and    demultiplexed before entering and after leaving the OADM    respectively. And there are not fundamental restrictions on the    wavelengths to be added or dropped.-   3) The coupling of the spectral channels into the output ports is    dynamically controlled by a servo-control assembly, rendering the    OADM less susceptible to environmental effects (such as thermal and    mechanical disturbances) and therefore more robust in performance.    By maintaining an optimal optical alignment, the optical losses    incurred by the spectral channels are also significantly reduced.-   4) The power levels of the spectral channels coupled into the output    ports can be dynamically managed according to demand, or maintained    at desired values (e.g., equalized at a predetermined value) by way    of the servo-control assembly. This spectral power-management    capability as an integral part of the OADM will be particularly    desirable in WDM optical networking applications.-   5) The use of free-space optics provides a simple, low loss, and    cost-effective construction. Moreover, the utilization of the    servo-control assembly effectively relaxes the requisite fabrication    tolerances and the precision of optical alignment during initial    assembly, enabling the OADM to be simpler and more adaptable in    structure, lower in cost and optical loss.-   6) The underlying OADM architecture allows a multiplicity of the    OADMs according to the present invention to be readily assembled    (e.g., cascaded) for WDM optical networking applications.

The novel features of this invention, as well as the invention itself,will be best understood from the following drawings and detaileddescription.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A-1D show a first embodiment of a wavelength-separating-routing(WSR) apparatus according to the present invention, and the modelingresults demonstrating the performance of the WSR apparatus;

FIGS. 2A-2C depict second and third embodiments of a WSR apparatusaccording to the present invention;

FIG. 3 shows a fourth embodiment of a WSR apparatus according to thepresent invention;

FIGS. 4A-4B show schematic illustrations of two embodiments of a WSR-Sapparatus comprising a WSR apparatus and a servo-control assembly,according to the present invention;

FIG. 5 depicts an exemplary embodiment of an optical add-dropmultiplexer (OADM) according to the present invention; and

FIG. 6 shows an alternative embodiment of an OADM according to thepresent invention.

DETAILED DESCRIPTION

In this specification and appending claims, a “spectral channel” ischaracterized by a distinct center wavelength and associated bandwidth.Each spectral channel may carry a unique information signal, as in WDMoptical networking applications.

FIG. 1A depicts a first embodiment of a wavelength-separating-routing(WSR) apparatus according to the present invention. By way of example toillustrate the general principles and the topological structure of awavelength-separating-routing (WSR) apparatus of the present invention,the WSR apparatus 100 comprises multiple input/output ports which may bein the form of an array of fiber collimators 110, providing an inputport 110-1 and a plurality of output ports 110-2 through 110-N (N≧3); awavelength-separator which in one form may be a diffraction grating 101;a beam-focuser in the form of a focusing lens 102; and an array ofchannel micromirrors 103.

In operation, a multi-wavelength optical signal emerges from the inputport 110-1. The diffraction grating 101 angularly separates themulti-wavelength optical signal into multiple spectral channels, whichare in turn focused by the focusing lens 102 into a spatial array ofdistinct spectral spots (not shown in FIG. 1A) in a one-to-onecorrespondence. The channel micromirrors 103 are positioned inaccordance with the spatial array formed by the spectral spots, suchthat each channel micromirror receives one of the spectral channels. Thechannel micromirrors 103 are individually controllable and movable,e.g., pivotable (or rotatable) under analog (or continuous) control,such that, upon reflection, the spectral channels are directed intoselected ones of the output ports 110-2 through 110-N by way of thefocusing lens 102 and the diffraction grating 101. As such, each channelmicromirror is assigned to a specific spectral channel, hence the name“channel micromirror”. Each output port may receive any number of thereflected spectral channels.

For purposes of illustration and clarity, only a selective few (e.g.,three) of the spectral channels, along with the input multi-wavelengthoptical signal, are graphically illustrated in FIG. 1A and the followingfigures. It should be noted, however, that there can be any number ofthe spectral channels in a WSR apparatus of the present invention (solong as the number of spectral channels does not exceed the number ofchannel mirrors employed in the system). It should also be noted thatthe optical beams representing the spectral channels shown in FIG. 1Aand the following figures are provided for illustrative purpose only.That is, their sizes and shapes may not be drawn according to scale. Forinstance, the input beam and the corresponding diffracted beamsgenerally have different cross-sectional shapes, so long as the angle ofincidence upon the diffraction grating is not equal to the angle ofdiffraction, as is known to those skilled in the art.

In the embodiment of FIG. 1A, it is preferable that the diffractiongrating 101 and the channel micromirrors 103 are placed respectively atthe first and second (i.e., the front and back) focal points (on theopposing sides) of the focusing lens 102. Such a telecentric arrangementallows the chief rays of the focused beams to be parallel to each otherand generally parallel to the optical axis. In this application, thetelecentric configuration further allows the reflected spectral channelsto be efficiently coupled into the respective output ports, therebyminimizing various translational walk-off effects that may otherwisearise. Moreover, the input multi-wavelength optical signal is preferablycollimated and circular in cross-section. The corresponding spectralchannels diffracted from the diffraction grating 101 are generallyelliptical in cross-section; they may be of the same size as the inputbeam in one dimension and elongated in the other dimension.

It is known that the diffraction efficiency of a diffraction grating isgenerally polarization-dependent. That is, the diffraction efficiency ofa grating in a standard mounting configuration may be considerablyhigher for P-polarization that is perpendicular to the groove lines onthe grating than for S-polarization that is orthogonal toP-polarization, especially as the number of groove lines (per unitlength) increases. To mitigate such polarization-sensitive effects, aquarter-wave plate 104 may be optically interposed between thediffraction grating 101 and the channel micromirrors 103, and preferablyplaced between the diffraction grating 101 and the focusing lens 102 asis shown in FIG. 1A. In this way, each spectral channel experiences atotal of approximately 90-degree rotation in polarization upontraversing the quarter-wave plate 104 twice. (That is, if a beam oflight has P-polarization when first encountering the diffractiongrating, it would have predominantly (if not all) S-polarization uponthe second encountering, and vice versa.) This ensures that all thespectral channels incur nearly the same amount of round-trippolarization dependent loss.

In the WSR apparatus 100 of FIG. 1A, the diffraction grating 101, by wayof example, is oriented such that the focused spots of the spectralchannels fall onto the channel micromirrors 103 in a horizontal array,as illustrated in FIG. 1B.

Depicted in FIG. 1B is a close-up view of the channel micromirrors 103shown in the embodiment of FIG. 1A. By way of example, the channelmicromirrors 103 are arranged in a one-dimensional array along thex-axis (i.e., the horizontal direction in the figure), so as to receivethe focused spots of the spatially separated spectral channels in aone-to-one correspondence. (As in the case of FIG. 1A, only threespectral channels are illustrated, each represented by a convergingbeam.) Let the reflective surface of each channel micromirror lie in thex-y plane as defined in the figure and be movable, e.g., pivotable (ordeflectable) about the x-axis in an analog (or continuous) manner. Eachspectral channel, upon reflection, is deflected in the y-direction(e.g., downward) relative to its incident direction, so to be directedinto one of the output ports 110-2 through 110-N shown in FIG. 1A.

As described above, a unique feature of the present invention is thatthe motion of each channel micromirror is individually and continuouslycontrollable, such that its position, e.g., pivoting angle, can becontinuously adjusted. This enables each channel micromirror to scan itscorresponding spectral channel across all possible output ports andthereby direct the spectral channel to any desired output port. Toillustrate this capability, FIG. 1C shows a plot of coupling efficiencyas a function of a channel micromirror's pivoting angle θ, provided by aray-tracing model of a WSR apparatus in the embodiment of FIG. 1A. Asused herein, the coupling efficiency for a spectral channel is definedas the ratio of the amount of optical power coupled into the fiber corein an output port to the total amount of optical power incident upon theentrance surface of the fiber (associated with the fiber collimatorserving as the output port). In the ray-tracing model, the input opticalsignal is incident upon a diffraction grating with 700 lines permillimeter at a grazing angle of 85 degrees, where the grating is blazedto optimize the diffraction efficiency for the “−1” order. The focusinglens has a focal length of 100 mm. Each output port is provided by aquarter-pitch GRIN lens (2 mm in diameter) coupled to an optical fiber(see FIG. 1D). As displayed in FIG. 1C, the coupling efficiency varieswith the pivoting angle θ, and it requires about a 0.2-degree change inθ for the coupling efficiency to become practically negligible in thisexemplary case. As such, each spectral channel may practically acquireany coupling efficiency value by way of controlling the pivoting angleof its corresponding channel micromirror. This is also to say thatvariable optical attenuation at the granularity of a single wavelengthcan be obtained in a WSR apparatus of the present invention. FIG. 1Dprovides ray-tracing illustrations of two extreme points on the couplingefficiency vs. θ curve of FIG. 1C: on-axis coupling corresponding toθ=0, where the coupling efficiency is maximum; and off-axis couplingcorresponding to θ=0.2 degrees, where the representative collimated beam(representing an exemplary spectral channel) undergoes a significanttranslational walk-off and renders the coupling efficiency practicallynegligible. All in all, the exemplary modeling results thus describeddemonstrate the unique capabilities of the WSR apparatus of the presentinvention.

FIG. 1A provides one of many embodiments of a WSR apparatus according tothe present invention. In general, the wavelength-separator is awavelength-separating means that may be a ruled diffraction grating, aholographic diffraction grating, an echelle grating, a dispersing prism,or other types of spectral-separating means known in the art. Thebeam-focuser may be a focusing lens, an assembly of lenses, or otherbeam-focusing means known in the art. The focusing function may also beaccomplished by using a curved diffraction grating as thewavelength-separator. The channel micromirrors may be provided bysilicon micromachined mirrors, reflective ribbons (or membranes), orother types of beam-deflecting elements known in the art. And eachmicromirror may be pivoted about one or two axes. What is important isthat the pivoting (or rotational) motion of each channel micromirror beindividually controllable in an analog manner, whereby the pivotingangle can be continuously adjusted so as to enable the channelmicromirror to scan a spectral channel across all possible output ports.The underlying fabrication techniques for micromachined mirrors andassociated actuation mechanisms are well documented in the art, see U.S.Pat. No. 5,629,790 for example. Moreover, a fiber collimator istypically in the form of a collimating lens (such as a GRIN lens) and aferrule-mounted fiber packaged together in a mechanically rigidstainless steel (or glass) tube. The fiber collimators serving as theinput and output ports may be arranged in a one-dimensional array, atwo-dimensional array, or other desired spatial pattern. For instance,they may be conveniently mounted in a linear array along a V-groovefabricated on a substrate made of silicon, plastic, or ceramic, ascommonly practiced in the art. It should be noted, however, that theinput port and the output ports need not necessarily be in close spatialproximity with each other, such as in an array configuration (although aclose packing would reduce the rotational range required for eachchannel micromirror). Those skilled in the art will know how to design aWSR apparatus according to the present invention, to best suit a givenapplication.

A WSR apparatus of the present invention may further comprise an arrayof collimator-alignment mirrors, for adjusting the alignment of theinput multi-wavelength optical signal and facilitating the coupling ofthe spectral channels into the respective output ports, as shown inFIGS. 2A-2B and 3.

Depicted in FIG. 2A is a second embodiment of a WSR apparatus accordingto the present invention. By way of example, WSR apparatus 200 is builtupon and hence shares a number of the elements used in the embodiment ofFIG. 1A, as identified by those labeled with identical numerals.Moreover, a one-dimensional array 220 of collimator-alignment mirrors220-1 through 220-N is optically interposed between the diffractiongrating 101 and the fiber collimator array 110. The collimator-alignmentmirror 220-1 is designated to correspond with the input port 110-1, foradjusting the alignment of the input multi-wavelength optical signal andtherefore ensuring that the spectral channels impinge onto thecorresponding channel micromirrors. The collimator-alignment mirrors220-2 through 220-N are designated to the output ports 110-2 through110-N in a one-to-one correspondence, serving to provide angular controlof the collimated beams of the reflected spectral channels and therebyfacilitating the coupling of the spectral channels into the respectiveoutput ports according to desired coupling efficiencies. Eachcollimator-alignment mirror may be rotatable about one axis, or twoaxes.

The embodiment of FIG. 2A is attractive in applications where the fibercollimators (serving as the input and output ports) are desired to beplaced in close proximity to the collimator-alignment mirror array 220.To best facilitate the coupling of the spectral channels into the outputports, arrays of imaging lenses may be implemented between thecollimator-alignment mirror array 220 and the fiber collimator array110, as depicted in FIG. 2B. By way of example, WSR apparatus 250 ofFIG. 2B is built upon and hence shares many of the elements used in theembodiment of FIG. 2A, as identified by those labeled with identicalnumerals. Additionally, first and second arrays 260, 270 of imaginglenses are placed in a 4-f telecentric arrangement with respect to thecollimator-alignment mirror array 220 and the fiber collimator array110. The dashed box 280 shown in FIG. 2C provides a top view of such atelecentric arrangement. In this case, the imaging lenses in the firstand second arrays 260, 270 all have the same focal length f. Thecollimator-alignment mirrors 220-1 through 220-N are placed at therespective first (or front) focal points of the imaging lenses in thefirst array 260. Likewise, the fiber collimators 110-1 through 110-N areplaced at the respective second (or back) focal points of the imaginglenses in the second array 270. And the separation between the first andsecond arrays 260, 270 of imaging lenses is 2f. In this way, thecollimator-alignment mirrors 220-1 through 220-N are effectively imagedonto the respective entrance surfaces (i.e., the front focal planes) ofthe GRIN lenses in the corresponding fiber collimators 110-1 through110-N. Such a telecentric imaging system substantially eliminatestranslational walk-off of the collimated beams at the output ports thatmay otherwise occur as the mirror angles change.

FIG. 3 shows a fourth embodiment of a WSR apparatus according to thepresent invention. By way of example, WSR apparatus 300 is built uponand hence shares a number of the elements used in the embodiment of FIG.2B, as identified by those labeled with identical numerals. In thiscase, the one-dimensional fiber collimator array 110 of FIG. 2B isreplaced by a two-dimensional array 350 of fiber collimators, providingfor an input-port and a plurality of output ports. Accordingly, theone-dimensional collimator-alignment mirror array 220 of FIG. 2B isreplaced by a two-dimensional array 320 of collimator-alignment mirrors,and first and second one-dimensional arrays 260, 270 of imaging lensesof FIG. 2B are likewise replaced by first and second two-dimensionalarrays 360, 370 of imagining lenses respectively. As in the case of theembodiment of FIG. 2B, the first and second two-dimensional arrays 360,370 of imaging lenses are placed in a 4-f telecentric arrangement withrespect to the two-dimensional collimator-alignment mirror array 320 andthe two-dimensional fiber collimator array 350. The channel micromirrors103 must be pivotable biaxially in this case (in order to direct itscorresponding spectral channel to any one of the output ports). As such,the WSR apparatus 300 is equipped to support a greater number of theoutput ports.

In addition to facilitating the coupling of the spectral channels intothe respective output ports as described above, the collimator-alignmentmirrors in the above embodiments also serve to compensate formisalignment (e.g., due to fabrication and assembly errors) in the fibercollimators that provide for the input and output ports. For instance,relative misalignment between the fiber cores and their respectivecollimating lenses in the fiber collimators can lead to pointing errorsin the collimated beams, which may be corrected for by thecollimator-alignment mirrors. For these reasons, thecollimator-alignment mirrors are preferably rotatable about two axes.They may be silicon micromachined mirrors, for fast rotational speeds.They may also be other types of mirrors or beam-deflecting elementsknown in the art.

To optimize the coupling of the spectral channels into the output portsand further maintain the optimal optical alignment against environmentaleffects such as temperature variations and mechanical instabilities overthe course of operation, a WSR apparatus of the present invention mayincorporate a servo-control assembly, for providing dynamic control ofthe coupling of the spectral channels into the respective output portson a channel-by-channel basis. A WSR apparatus incorporating aservo-control assembly is termed a WSR-S apparatus, thereinafter in thisspecification.

FIG. 4A depicts a schematic illustration of a first embodiment of aWSR-S apparatus according to the present invention. The WSR-S apparatus400 comprises a WSR apparatus 410 and a servo-control assembly 440. TheWSR 410 may be in the embodiment of FIG. 1A, or any other embodiment inaccordance with the present invention. The servo-control assembly 440includes a spectral monitor 460, for monitoring the power levels of thespectral channels coupled into the output ports 420-1 through 420-N ofthe WSR apparatus 410. By way of example, the spectral monitor 460 iscoupled to the output ports 420-1 through 420-N by way of fiber-opticcouplers 420-1-C through 420-N-C, wherein each fiber-optic couplerserves to tap off a predetermined fraction of the optical signal in thecorresponding output port. The servo-control assembly 440 furtherincludes a processing unit 470, in communication with the spectralmonitor 460 and the channel micromirrors 430 of the WSR apparatus 410.The processing unit 470 uses the power measurements from the spectralmonitor 460 to provide feedback control of the channel micromirrors 430on an individual basis, so as to maintain a desired coupling efficiencyfor each spectral channel into a selected output port. As such, theservo-control assembly 440 provides dynamic control of the coupling ofthe spectral channels into the respective output ports on achannel-by-channel basis and thereby manages the power levels of thespectral channels coupled into the output ports. The power levels of thespectral channels in the output ports may be dynamically managedaccording to demand, or maintained at desired values (e.g., equalized ata predetermined value) in the present invention. Such a spectralpower-management capability is essential in WDM optical networkingapplications, as discussed above.

FIG. 4B depicts a schematic illustration of a second embodiment of aWSR-S apparatus according to the present invention. The WSR-S apparatus450 comprises a WSR apparatus 480 and a servo-control assembly 490. Inaddition to the channel micromirrors 430 (and other elements identifiedby the same numerals as those used in FIG. 4A), the WSR apparatus 480further includes a plurality of collimator-alignment mirrors 485, andmay be configured according to the embodiment of FIGS. 2A, 2B, 3, or anyother embodiment in accordance with the present invention. By way ofexample, the servo-control assembly 490 includes the spectral monitor460 as described in the embodiment of FIG. 4A, and a processing unit495. In this case, the processing unit 495 is in communication with thechannel micromirrors 430 and the collimator-alignment mirrors 485 of theWSR apparatus 480, as well as the spectral monitor 460. The processingunit 495 uses the power measurements from the spectral monitor 460 toprovide dynamic control of the channel micromirrors 430 along with thecollimator-alignment mirrors 485, so to maintain the couplingefficiencies of the spectral channels into the output ports at desiredvalues.

In the embodiment of FIG. 4A or 4B, the spectral monitor 460 may be oneof spectral power monitoring devices known in the art that is capable ofdetecting the power levels of spectral components in a multi-wavelengthoptical signal. Such devices are typically in the form of awavelength-separating means (e.g., a diffraction grating) that spatiallyseparates a multi-wavelength optical signal by wavelength intoconstituent spectral components, and one or more optical sensors (e.g.,an array of photodiodes) that are configured such to detect the powerlevels of these spectral components. The processing unit 470 in FIG. 4A(or the processing unit 495 in FIG. 4B) typically includes electricalcircuits and signal processing programs for processing the powermeasurements received from the spectral monitor 460 and generatingappropriate control signals to be applied to the channel micromirrors430 (and the collimator-alignment mirrors 485 in the case of FIG. 4B),so to maintain the coupling efficiencies of the spectral channels intothe output ports at desired values. The electronic circuitry and theassociated signal processing algorithm/software for such processing unitin a servo-control system are known in the art. A skilled artisan willknow how to implement a suitable spectral monitor along with anappropriate processing unit to provide a servo-control assembly in aWSP-S apparatus according to the present invention, for a givenapplication.

The incorporation of a servo-control assembly provides additionaladvantages of effectively relaxing the requisite fabrication tolerancesand the precision of optical alignment during initial assembly of a WSRapparatus of the present invention, and further enabling the system tocorrect for shift in the alignment over the course of operation. Bymaintaining an optimal optical alignment, the optical losses incurred bythe spectral channels are also significantly reduced. As such, the WSR-Sapparatus thus constructed is simpler and more adaptable in structure,more robust in performance, and lower in cost and optical loss.Accordingly, the WSR-S (or WSR) apparatus of the present invention maybe used to construct a variety of optical devices and utilized in manyapplications.

For instance, by directing the spectral channels into the output portsin a one-channel-per-port fashion and coupling the output ports of aWSR-S (or WSR) apparatus to an array of optical sensors (e.g.,photodiodes), or a single optical sensor that is capable of scanningacross the output ports, a dynamic and versatile spectral power monitor(or channel analyzer) is provided, which would be highly desired in WDMoptical networking applications. Moreover, a novel class of opticaladd-drop multiplexers (OADMs) may be built upon the WSR-S (or WSR)apparatus of the present invention, as exemplified in the followingembodiments.

FIG. 5 depicts an exemplary embodiment of an optical add-dropmultiplexer (OADM) according to the present invention. By way ofexample, OADM 500 comprises a WSR-S (or WSR) apparatus 510 and anoptical combiner 550. An input port 520 of the WSR-S apparatus 510transmits a multi-wavelength optical signal. The constituent spectralchannels are subsequently separated and routed into a plurality ofoutput ports, including a pass-through port 530 and one or more dropports 540-1 through 540-N (N≧1). The pass-through port 530 may receiveany number of the spectral channels (i.e., the pass-through spectralchannels). Each drop port may also receive any number of the spectralchannels (i.e., the drop spectral channels). The pass-through port 530is optically coupled to the optical combiner 550, which serves tocombine the pass-through spectral channels with one or more add spectralchannels provided by one or more add ports 560-1 through 560-M (M≧1).The combined optical signal is then routed into an existing port 570,providing an output multi-wavelength optical signal.

In the above embodiment, the optical combiner 550 may be a K×1 (K≧2)broadband fiber-optic coupler, wherein there are K input-ends and oneoutput-end. The pass-through spectral channels and the add spectralchannels are fed into the K input-ends (e.g., in a one-to-onecorrespondence) and the combined optical signal exits from theoutput-end of the K×1 fiber-optic coupler as the output multi-wavelengthoptical signal of the system. Such a multiple-input coupler also servesthe purpose of multiplexing a multiplicity of add spectral channels tobe coupled into the OADM 500. If the power levels of the spectralchannels in the output multi-wavelength optical signal are desired to beactively managed, such as being equalized at a predetermined value, twospectral monitors may be utilized. As a way of example, the firstspectral monitor may receive optical signals tapped off from thepass-through port 530 and the drop ports 540-1 through 540-N (e.g., byway of fiber-optic couplers as depicted in FIG. 4A or 4B). The secondspectral monitor receives optical signals tapped off from the exitingport 570. A servo-control system may be constructed accordingly formonitoring and controlling the pass-through, drop and add spectralchannels. As such, the embodiment of FIG. 5 provides a versatile opticaladd-drop multiplexer in a simple and low-cost assembly, while providingmultiple physically separate drop/add ports in a dynamicallyreconfigurable fashion.

FIG. 6 depicts an alternative embodiment of an optical add-dropmultiplexer (OADM) according to the present invention. By way ofexample, OADM 600 comprises a first WSR-S apparatus 610 opticallycoupled to a second WSR-S apparatus 650. Each WSR-S apparatus may be inthe embodiment of FIG. 4A or 4B. (A WSR apparatus of the embodiment ofFIG. 1A, 2A, 2B, or 3 may be alternatively implemented.) The first WSR-Sapparatus 610 includes an input port 620, a pass-through port 630, andone or more drop ports 640-1 through 640-N (N≧1). The pass-throughspectral channels from the pass-through port 630 are further coupled tothe second WSR-S apparatus 650, along with one or more add spectralchannels emerging from add ports 660-1 through 660-M (M≧1). In thisexemplary case, the pass-through port 630 and the add ports 660-1through 660-M constitute the input ports for the second WSR-S apparatus650. By way of its constituent wavelength-separator (e.g., a diffractiongrating) and channel micromirrors (not shown in FIG. 6), the secondWSR-S apparatus 650 serves to multiplex the pass-through spectralchannels and the add spectral channels, and route the multiplexedoptical signal into an exiting port 770 to provide an output signal ofthe system.

In the embodiment of FIG. 6, one WSR-S apparatus (e.g., the first WSR-Sapparatus 610) effectively performs dynamic drop function, whereas theother WSR-S apparatus (e.g., the second WSR-S apparatus 650) carries outdynamic add function. And there are essentially no fundamentalrestrictions on the wavelengths that can be added or dropped (other thanthose imposed by the overall communication system). Moreover, theunderlying OADM architecture thus presented is intrinsically scalableand can be readily extended to any number of cascaded WSR-S (or WSR)systems, if so desired for performing intricate add and drop functions.Additionally, the OADM of FIG. 6 may be operated in reverse direction,by using the input ports as the output ports, the drop ports as the addports, and vice versa.

Those skilled in the art will recognize that the aforementionedembodiments provide only two of many embodiments of a dynamicallyreconfigurable OADM according to the present invention. Those skilled inthe art will also appreciate that various changes, substitutions, andalternations can be made herein without departing from the principlesand the scope of the invention as defined in the appended claims.Accordingly, a skilled artisan can design an OADM in accordance with theprinciples of the present invention, to best suit a given application.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions, andalternations can be made herein without departing from the principlesand the scope of the invention. Accordingly, the scope of the presentinvention should be determined by the following claims and their legalequivalents.

1. An optical add-drop apparatus comprising an input port for an inputmulti-wavelength optical signal having first spectral channels; one ormore other ports for second spectral channels; an output port for anoutput multi-wavelength optical signal; a wavelength-selective devicefor spatially separating said spectral channels; and a spatial array ofbeam-deflecting elements positioned such that each element receives acorresponding one of said spectral channels, each of said elements beingindividually and continuously controllable to reflect its correspondingspectral channel to a selected one of said ports.
 2. The opticaladd-drop apparatus of claim 1 further comprising a control unit forcontrolling each of said beam-deflecting elements.
 3. The opticaladd-drop apparatus of claim 2, wherein the control unit furthercomprises a servo-control assembly, including a spectral monitor formonitoring power levels of selected ones of said spectral channels, anda processing unit responsive to said power levels for controlling saidbeam-deflecting elements.
 4. The optical add-drop apparatus of claim 3,wherein said servo-control assembly maintains said power levels atpredetermined values.
 5. The optical add-drop apparatus of claim 2,wherein the control unit controls said beam-deflecting elements todirect selected ones of said first spectral channels to one or more ofsaid second ports to be dropped as second spectral channels from saidoutput multi-wavelength optical signal.
 6. The optical add-dropapparatus of claim 2, wherein the control unit controls saidbeam-deflecting elements to direct selected ones of said second spectralchannels to said output port to be added to said output multi-wavelengthoptical signal.
 7. The optical add-drop apparatus of claim 1 furthercomprising alignment mirrors for adjusting alignment of said input andoutput multi-wavelength optical signals and said second spectralchannels with said wavelength-selective device.
 8. The optical add-dropapparatus of claim 7 further comprising collimators associated with saidalignment mirrors, and imaging lenses in a telecentric arrangement withsaid alignment mirrors and said collimators.
 9. The optical add-dropapparatus of claim 1, wherein said wavelength selective device furthercombines selected ones of said spectral channels reflected from saidbeam-deflecting elements to form said output multi-wavelength opticalsignal.
 10. The optical add-drop apparatus of claim 1, wherein said oneor more other ports comprise an add port and a drop port forrespectively adding second and dropping first spectral channels.
 11. Theoptical add-drop apparatus of claim 1 further comprising a beam-focuserfor focusing said separated spectral channels onto said beam deflectingelements.
 12. The optical add-drop apparatus of claim 1, wherein saidwavelength-selective device comprises a device selected from the groupconsisting of ruled diffraction gratings, holographic diffractiongratings, echelle gratings, curved diffraction gratings, and dispersingprisms.
 13. The optical add-drop apparatus of claim 1, wherein saidbeam-deflecting elements comprise micromachined mirrors.
 14. The opticaladd-drop apparatus of claim 1, wherein said beam-deflecting elementscomprise reflective membranes.
 15. An optical add-drop apparatus,comprising an input port for an input multi-wavelength optical signalhaving multiple spectral channels; an output port for an outputmulti-wavelength optical signal; one or more drop ports for selectedspectral channels dropped from said multi-wavelength optical signal; awavelength-selective device for spatially separating said multiplespectral channels; and a spatial array of beam-deflecting elementspositioned such that each element receives a corresponding one of saidspectral channels, each of said elements being individually andcontinuously controllable to reflect its corresponding spectral channelto a selected one of said ports, whereby a subset of said spectralchannels is directed to said drop ports.
 16. An optical add-dropapparatus, comprising an input port for an input multi-wavelengthoptical signal having multiple spectral channels; an output port for anoutput multi-wavelength optical signal; one or more add ports forselected spectral channels to be added to said output multi-wavelengthoptical signal; a wavelength-selective device for reflecting saidmultiple and said selected spectral channels; and a spatial array ofbeam-deflecting elements positioned such that each element receives acorresponding one of said spectral channels, each of said elements beingindividually and continuously controllable to reflect its correspondingspectral channel to a selected one of said ports, whereby said spectralchannels from said add ports are selectively provided to said outputport.
 17. A method of performing dynamic add and drop in a WDM opticalnetwork, comprising separating an input multi-wavelength optical signalinto spectral channels; imaging each of said spectral channels onto acorresponding beam-deflecting element; and controlling dynamically andcontinuously said beam-deflecting elements so as to combine selectedones of said spectral channels into an output multi-wavelength opticalsignal.
 18. The method of claim 17, wherein said selected ones of saidspectral channels comprises a subset of said spectral channels, suchthat other non-selected ones of said spectral channels are dropped fromsaid output multi-wavelength optical signal.
 19. The method of claim 18,wherein said controlling comprises reflecting said non-selected ones ofsaid spectral channels to one or more drop ports.
 20. The method ofclaim 17 further comprising imaging other spectral channels onto othercorresponding beam-deflecting elements, and controlling dynamically andcontinuously said other beam-deflecting elements so as to combine saidother spectral channels with said selected ones of said spectralchannels into said output multi-wavelength optical signal.
 21. Themethod of claim 17, wherein said imaging comprises focusing saidspectral channels onto said beam-deflecting elements.
 22. The method ofclaim 17 further comprising monitoring a power level in one or more ofsaid selected ones of said spectral channels, and controlling analignment between said input multi-wavelength optical signal andcorresponding beam-deflecting elements in response to said monitoring.